The present invention relates generally to telephone switching systems, and more particularly to a least cost routing technique for outgoing calls through an ISDN (integrated Services Digital Network) network.
According to conventional least cost routing schemes available for ISDN switching systems, digit conversion data and least cost route numbers are stored in a memory. In response to receipt of a call setup message from a calling party, the called party number contained in the message is used as an address pointer to retrieve digit conversion data and a least-cost route number from the memory. The header portion of the called party number is converted according to the retrieved digit conversion data and a least cost route is established through a public switched ISDN network according to the retrieved route number. Different types of customer premises equipment can be connected through an ISDN interface standardized by a CCITT Recommendation to a single subscriber loop. The type of such equipment is identified by a 5-bit information transfer capability identifier contained in the call setup message. However, the current switching system makes no distinction between different information transfer capabilities when determining a least cost route. Therefore, it is impossible to treat outgoing calls according to their information transfer capabilities when one wishes to take advantage of least cost routing only from voice traffic, while preventing other traffic such as facsimile and data from using this feature.